When to use MMs proposition 2 formula

Good Morning

I understand when we should use WACC, when we should ungear, regear and when to use APV. that all makes sense

I also understand we can use CAPM and DVM to get the Ke. However, i dont really understand when we would use the lovely MM proposition 2 formula

In your lectures you mentioned, its very rarely examined, and we can of course use CAPM as a way to go around it. however, how will i know the signs of when to use it?